1. Field of the Invention
Aspects of the present invention relate to flux control systems and, more particularly, to flux control systems for use in motion control applications. Aspects of the present invention also relate to the control of electric machines, such as switched reluctance machines, permanent magnet machines and hybrids thereof.
2. Description of Related Art
In many electromagnetic systems, the transfer of energy from one component of the system to another is critical to proper operation of the system. In many electromagnetic systems, this transfer of energy is accomplished by appropriately energizing one component of the system to establish a magnetic flux that interacts with another component of the system to transfer energy from the energized component to the other component. Despite the fact that the energy transfer is accomplished by the flux, in known electromagnetic systems the flux of the system is not directly controlled. Instead, the current and/or voltage applied to the energized member is controlled and, based on assumed relationships between current, voltage and flux, it is assumed that the control of the current and/or voltage based on the assumed relationships will produce the appropriate flux. Control of current and/or voltage is typically implemented, at least in part, because the prior art has not provided an efficient, low-cost, and easily implemented system for directly controlling flux in an electromagnetic system.
One drawback of current and/or voltage control systems as described above is that the relationships between current, voltage and flux are not easily represented mathematically and vary in a non-linear manner depending on a variety of variables. For example, the particular characteristics of each piece of magnetic material in a system will result in voltage, current and flux relationships that vary from one system to another and, even within a given system, from one section of the system to another. Because of these differing voltage, current and flux relationships, it is difficult to accurately and properly control the currents and/or voltages to produce the desired flux and, thus, the desired energy transfer. As such, the prior art is limited in its ability to provide an electromagnetic system in which flux is directly controlled.
The lack of an appropriate flux control system in the prior art is particularly noticeable in electromagnetic systems where it is desired to finally control the force exerted by one component of the system on another component of the system. In such systems, the actual force produced by the system is related to the flux established by the energized component of the system. As described above, however, because the prior art cannot directly and finely control flux, it cannot, therefore, finely control the force produced by such systems. The inability of the prior art to finely control the forces established in an electromagnetic system is particularly acute in applications where the movement of at least one component of the system must be precisely controlled.
The typical switched reluctance machine comprises a rotor defining rotor poles, a stator defining stator poles, and a set of windings arranged in relation to the stator poles to define one or more phases. In a reluctance machine, energization of one or more phase windings sets up a magnetic flux in the associated stator poles, urging the rotor into a position of minimum reluctance. Timing the sequential energization of the windings according to rotor position induces rotor movement. Switched reluctance machines are well known. More detail is provided in the paper xe2x80x98The Characteristics, Design and Applications of Switched Reluctance Motors and Drivesxe2x80x99 by Stephenson and Blake, presented at the PCIM ""93 Conference and Exhibition at Nurnberg, Germany, Jun. 21-24, 1993, which is incorporated herein by reference. As is well known in the art, these machines can be operated as motors or generators simply by altering the timing of the application of the excitation to the phase windings.
As explained in the Stephenson and Blake paper, the method of torque production in a switched reluctance machine is quite different from that in conventional machines, e.g. induction or synchronous machines, which are operated by rotating waves of magneto-motive force (mmf) and in which the torque is produced by the interaction of a magnetic field with a current flowing in a conductor. Such machines are known as xe2x80x98electromagneticxe2x80x99 machines and encompass, e.g., so-called brushless DC machines in which the current is in stator coils and the field is produced by permanent magnets on the rotor. These machines require the use of permanent magnets.
By contrast, switched reluctance machines are purely xe2x80x98magneticxe2x80x99 machines, where the torque is produced solely by the magnetic field as the reluctance of the magnetic circuit changes. It follows that the methods of controlling the two types of machine are typically quite different, since the control is related to the method of torque production. In general, the control methods used for conventional, sinusoidally fed machines have been considered quite inappropriate for switched reluctance machines.
FIG. 1 shows a typical switched reluctance machine in cross section. In this example, the stator 10 has six stator poles 12, and the rotor 14 has four rotor poles 16. Each stator pole carries a coil 18. The coils on diametrically opposite poles are connected in series to provide three phase windings. Only one phase winding is shown, for clarity. The control of the switched reluctance machine can be achieved in a variety of ways. The machine could be controlled in an open-loop fashion, i.e. as commonly used for stepping motors. In this regime, the phase windings in the machine are sent pulses in turn and it is assumed that the rotor lines up with each pair of stator poles in turn, i.e. the position of minimum reluctance for that phase which is excited. Of course, because the system is open-loop, there are no means of knowing if the rotor has moved or not. To remove this uncertainty, it is conventional to use a rotor position detection scheme of some sort which provides a signal representative of rotor position. The excitation can then be applied as a function of the position. Such machines are often referred to as xe2x80x9crotor position switched machinesxe2x80x9d.
Since current in the windings is relatively easy to measure, closed-loop current control is commonly accomplished by monitoring and controlling the energizing current in the windings. However, the desired output of the machine is usually torque, position or speed, and current has a highly non-linear relationship to all of these. The result is that current control techniques generally have inaccuracies in the output, such as torque ripple, position error or speed error.
A typical switched reluctance drive is shown in FIG. 2. In this example, the machine 36 corresponds to that shown in FIG. 1. The three phase windings A, B and C are switched onto a d.c. supply V by a set of power electronic switches 48. The moments at which the switches operate are determined by the controller 38, which may be implemented either in hardware or in the software of a microcontroller or digital signal processor. The firing signals are sent to the switches via a data bus 46. Closed loop current feedback is provided by sensing the phase currents by one or more current sensors 44 and feeding back signals proportional to phase current. The control algorithms often include a proportional (P), proportional-plus-integral (P+I), time optimal, feedback linearized, proportional/integral/derivative (PID) function, or one of many others as is well understood in the art. It is also common for an outer control loop of position or speed to be provided by feeding back a rotor position signal from a position detector 40.
In operation, a current demand iD on line 42 is provided to the controller and this regulates the current in the windings, according to the particular control scheme adopted, to produce the desired output from the machine. Those skilled in the art will be familiar with the many variations of current controllers that exist, each of which has its own merits, but all of them suffer from the problems of non-linearity between the controlled variable and the machine output described above.
In the paper xe2x80x98Torque Control of Switched Reluctance Drivesxe2x80x99 by P. G. Barrass and B. C. Mecrow, ICEM""96 Proceedings, International Conference on Electrical Machines, Sep. 10-12, 1996, Vigo, Spain, Vol 1, pp 254-259, incorporated herein by reference, there is a proposal to provide a torque control by reference to flux linkage reference waveforms using a look-up table that stores fixed values of flux ramps for co-ordinates of supply voltage, phase current and rotor position. The flux values and co-ordinates are specific to a particular motor. At any instant the prestored values of flux and torque are chosen from measurements of phase current and the stored machine data. There is a fixed relationship between the monitored variables and the values of the flux waveforms in the look-up table that are used to produce an output for a given motor.
According to aspects of the invention, a flux control system includes a flux controller, adapted to receive a flux command as an input and to provide a phase coil energization signal as an output, an electromagnetic system adapted to receive the phase coil energization signal, the electromagnetic system comprising at least one phase coil, and a flux observer adapted to provide a feedback signal to the flux controller, the feedback signal corresponding to flux in the electromagnetic system, wherein the phase coil energization signal provided by the flux controller energizes the electromagnetic system such that flux in the electromagnetic system follows the flux command. The electromagnetic system can include an electromagnetic actuator. The electromagnetic system can include a plurality of phase coils, and the energization signal is in the form of an energization vector that includes separate energization signals for each of the phase coils. The electromagnetic system also can include a single phase coil. The flux observer can include a Hall-effect probe, such as a thin-film Hall-effect device. The flux observer also can include a Gauss meter.
According to additional aspects of the invention, an electrical machine includes a rotor, a stator operably coupled with the rotor, at least one phase winding operably coupled with the rotor and stator and arranged to establish flux in a magnetic circuit in the machine, and a flux observer adapted to produce a signal indicative of flux-causing voltage across the at least one phase winding. The flux observer can include a Hall-effect device. The flux observer is arranged in a flux path of the machine. The flux observer is operable to produce the signal as a voltage or current directly proportional to the flux. The machine is a brushless electrical machine, according to aspects of the invention, the rotor is an unmagnetized rotor, and the stator is an unmagnetized stator. The observer can include a search coil arranged in relation to the magnetic circuit to produce the signal indicative of the flux-causing voltage. The machine is a switched reluctance machine, according to aspects of the invention.
According to additional aspects of the invention, an electrical drive system includes an electrical machine having a rotor, a stator and at least one phase winding arranged to establish flux in a magnetic circuit in the machine, an observer adapted to produce a feedback signal proportional to flux-causing voltage across the or each phase winding, and a flux controller having an input signal representing demanded output of the machine, which controller is responsive to the input signal and the feedback signal to produce control signals for actuating at least one switch to control the flux in the at least one phase winding. The observer includes a transducer operably coupled with the or each phase winding, includes a search coil, is part of a flux estimator adapted to produce a flux signal proportional to flux in the or each phase winding from the feedback signal, and/or is operably coupled with a voltage model of the machine for producing the feedback signal. The estimator includes a current model of the machine arranged to receive signals representing phase current and rotor position and is operable to produce a flux estimate for the or each phase winding therefrom. The estimator includes at least one comparator for producing a current model error signal from the flux estimate and the feedback signal.
The observer is operably coupled with a voltage model of the machine for producing the feedback signal, according to aspects of the invention, and further includes at least one adder for summing output of the voltage model and differentiated output of the current model to produce the feedback signal. The estimator includes a current model controller arranged to apply a control law function to output of the current model, the current model controller having a response to machine speed such that a current model output signal is increasingly attenuated with increasing machine speed above a predetermined machine speed. The observer is operably coupled with a voltage model of the machine for producing the feedback signal, and the system is adapted to cause output of the current model to dominate output of the voltage model at relatively low machine speeds, and to cause output of the voltage model to dominate output of the current model at relatively high machine speeds. The electrical machine is a switched reluctance machine, according to aspects of the invention.
According to additional aspects of the invention, a method of controlling an electrical machine having a rotor, a stator and at least one phase winding includes arranging a transducer in a magnetic circuit of the machine to produce a flux signal indicative of the flux in the at least one phase winding, producing an input signal representing a demanded output of the machine, and controlling energization of the at least one phase winding in response to the input signal and the flux signal. The electrical machine is a switched reluctance machine, according to aspects of the invention.
These and other aspects and advantages according to the invention will be apparent upon reading the remainder of this application.